You are not alone

When I had Hazel, I had so many doubts. Doubts about myself as a mother, my ability to do the right thing, my efforts to care for this vulnerable little person. My doubts were ever present but not always at the forefront of my thoughts. It was like having an old, hideously ugly great aunt staying with me. I didn't want to interact with her or really even look her way but I had to, she's a part of my family and therefore a part of me. And sometimes she was so demanding! Right in my face, all warty and wrinkly with foul smelling breath. She would block out everything else, all other thoughts, desires, actions. I would be completely consumed by her presence. But sometimes I could keep her in my peripheral vision, just on the outskirts of my awareness. And I would think, "This isn't so bad, I can handle this." As long as I didn't look at her, I could almost forget she was there. But late at night or after a day filled with crying, pooping and isolation, she would be right in my face again, haunting me with racing thoughts and dark feelings.
If you have struggled with this, even if you don't struggle all the time, I'm here to tell you two things: you are not alone and there is help for you. 
Before I had kids, I had no idea how taxing a newborn could be. I'd never been so sleep deprived, so stretched to my limits and so worried. I would see other moms taking care of numerous children and think I was inferior in some way because one baby was so hard for me. In America we value independence almost above all else and sometimes it can be to our detriment. I'm realizing now that it really does take a village to raise a child. I cannot possibly be everything for my kids, they need other responsible adults in their lives in order to grow up and be well adjusted people. I have loved living by family, it has eased some of the immense burden of child rearing. If you don't live by your family, I suggest you build a network of friends who can love and support you and your children. We really do need each other. Tim always says economies of scale work well with child care. Also, don't forget you have a loving Heavenly Father who is rooting for you and will send angels down to buoy you up. (Motherhood: An Eternal Partnership with God.) I remember a talk by Elder Holland in which he said "Christ knows better than all others that the trials of life can be very deep and we are not shallow people if we struggle with them." (An High Priest of Good Things to Come, Elder Holland, October 1999) He is there for you always, pray to Him for help and He will not let you down.
In regards to getting help, you have to ask. Nobody can read your thoughts (you probably wouldn't want them to anyway) and you can seem surprisingly put together even when you're coming apart at the seams inside. You aren't doing anybody any favors by pretending you're fine when you're really struggling. So be honest when people ask you how you're doing. Say things like "I'm not getting any sleep and nursing is so painful." I've been surprised how empathetic and helpful people can be when they know you're suffering. When people offer to help you, tell them what they can do. I used to hate it when people would say "Call me if you need anything." Because I didn't think they really meant it. Well, most of the time they really do want you to call, they may want to help but aren't sure how. Don't feel like you're putting anybody out by asking someone to come over and hold your baby while you shower or take a nap or get out of the house. People love babies, especially other people's babies. And don't worry that something will go wrong, most women have raised children to adulthood with little to no tragedies. Your baby will not stop breathing the minute you leave the house, she probably won't even notice you are gone. Plan breaks when you are doing well. I'm terrible at this. I wait until I can't stand one more minute of crying, pulling, demanding baby behavior and then I try to call someone. It's incredibly frustrating when you're so frazzled and you can't get a hold of anybody to come help you. So plan to take time for yourself before you're at your wits end. If you have more than one kid, farm them out to several people. They'll love playing with friends and you'll be a better mom because of the break. And don't worry about not being able to reciprocate child care. I felt guilty about this for months. I didn't want to trade with any of my friends because I was having a hard enough time with my own children, let alone adding some one else's kids in the mix. Also, I didn't trust myself with other people's kids, I wouldn't want someone struggling with postpartum depression to watch my kids so I got comfortable being on the receiving end of service. I know my turn will come again to be on the giving end of service but for now, I need a lot of help.
I hope you can benefit from my insights, experiences and testimony of Jesus Christ. I love reading your comments and feeling your support. Thanks for reading my thoughts.
PS- I just found this Ted Talk that speaks volumes about depression. It's by a teenager named Kevin Breel and it's entitled Confessions of a Depressed Comic.